"Tegami" is a fantasy anime series that revolves around the journey of a young girl named Lag Seeing, who aspires to become a Letter Bee, a prestigious courier responsible for delivering letters across a dangerous and fragmented world. The story is set in Amberground, a land perpetually shrouded in darkness, divided into three regions separated by impassable barriers. The only light comes from an artificial sun, and the world is plagued by monstrous creatures known as Gaichuu, which threaten travelers and couriers alike.

Lag’s motivation to become a Letter Bee stems from his childhood, when he was delivered to his foster mother by a kind-hearted Letter Bee named Gauche Suede. Gauche’s compassion and dedication left a lasting impression on Lag, inspiring him to follow in his footsteps. However, Gauche mysteriously disappears, and Lag later discovers that he has lost his memories and become a rogue operative known as "Reverse." This revelation drives Lag to uncover the truth behind Gauche’s transformation while continuing his duties as a Letter Bee.

The narrative explores themes of connection, memory, and the power of communication. Letters in this world are not merely physical objects but vessels of emotion, often containing the heartfelt thoughts and memories of the senders. Lag’s journey is marked by his encounters with various individuals, including his loyal dingo, Niche, a spirited girl with mysterious origins, and her companion, Steak, a shapeshifting creature. Together, they navigate the challenges of their missions while confronting the darker forces at play in Amberground.

As Lag progresses in his role, he uncovers deeper conspiracies involving the government and the true nature of the Gaichuu. The series delves into the complexities of human relationships, the weight of responsibility, and the resilience required to face a world filled with uncertainty and danger. Through its episodic structure and overarching plot, "Tegami" balances moments of heartfelt emotion with action and intrigue, creating a compelling narrative that highlights the importance of hope and perseverance in a fractured world.